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BACKGROUND: Prostate cancer is a major public health problem with considerable uncertainties about the effectiveness of population screening and treatment options. We report the study design, participant sociodemographic and clinical characteristics, and the initial results of the testing and diagnostic phase of the Prostate testing for cancer and Treatment (ProtecT) trial, which aims to investigate the effectiveness of treatments for localised prostate cancer. METHODS: In this randomised phase 3 trial, men aged 50-69 years registered at 337 primary care centres in nine UK cities were invited to attend a specialist nurse appointment for a serum prostate-specific antigen (PSA) test. Prostate biopsies were offered to men with a PSA concentration of 3·0 μg/L or higher. Consenting participants with clinically localised prostate cancer were randomly assigned to active monitoring (surveillance strategy), radical prostatectomy, or three-dimensional conformal external-beam radiotherapy by a computer-generated allocation system. Randomisation was stratified by site (minimised for differences in participant age, PSA results, and Gleason score). The primary endpoint is prostate cancer mortality at a median 10-year follow-up, ascertained by an independent committee, which will be analysed by intention to treat in 2016. This trial is registered with ClinicalTrials.gov, number NCT02044172, and as an International Standard Randomised Controlled Trial, number ISRCTN20141297. FINDINGS: Between Oct 1, 2001, and Jan 20, 2009, 228,966 men were invited to attend an appointment with a specialist nurse. Of the invited men, 100,444 (44%) attended their initial appointment and 82,429 (82%) of attenders had a PSA test. PSA concentration was below the biopsy threshold in 73,538 (89%) men. Of the 8566 men with a PSA concentration of 3·0-19·9 μg/L, 7414 (87%) underwent biopsies. 2896 men were diagnosed with prostate cancer (4% of tested men and 39% of those who had a biopsy), of whom 2417 (83%) had clinically localised disease (mostly T1c, Gleason score 6). With the addition of 247 pilot study participants recruited between 1999 and 2001, 2664 men were eligible for the treatment trial and 1643 (62%) agreed to be randomly assigned (545 to active monitoring, 545 to radiotherapy, and 553 to radical prostatectomy). Clinical and sociodemographic characteristics of randomly assigned participants were balanced across treatment groups. INTERPRETATION: The ProtecT trial randomly assigned 1643 men with localised prostate cancer to active monitoring, radiotherapy, or surgery. Participant clinicopathological features are more consistent with contemporary patient characteristics than in previous prostate cancer treatment trials. FUNDING: UK National Institute for Health Research Health Technology Assessment Programme.